1. Field of the Invention
The present invention relates to apparatus for measuring the level in an oil reservoir of a motor and more particularly to such apparatus for measuring the dynamic oil level in a reservoir surrounding a bearing on a motor shaft. More particularly, the invention relates to apparatus for monitoring the level of lubricating oil in the oil pot of a motor, the invention having particular application to reactor coolant pump motors in nuclear power plants.
2. Description of Related Art
The reactor coolant pump motors drive the reactor coolant pumps which are part of the primary reactor coolant system in a nuclear power plant. The motors are typically arranged with vertical shafts. The bearing and lubrication system of a vertical motor is usually contained in two separate oil pots (also referred to as oil reservoirs). An upper oil pot contains the upper guide or radial bearings and the total thrust bearing system. The lower oil pot contains the lower guide or radial bearings. Each of these oil pots is typically provided with cooling coils for carrying cooling water to dissipate the heat which is generated by the bearing systems.
The design of the oil pots is such that the oil level within the pot should be monitored during operation to ensure that the oil level is not rising above or falling below the expected levels. A rising level might indicate, for example, a water leak within the cooling coils which results in water entering the pot and mixing with the oil. If such a situation were to persist, the lubricating ability of the oil would be sharply diminished and, more importantly, the oil/water mixture would overflow the pot and migrate toward the hot reactor coolant pump, where a fire could result.
A falling oil level would be indicative of a leak in the oil pot system which allows oil to escape from the pots. If this situation persists, the level of oil in the pot would drop below the level where the oil lubricates the bearings and thus result in severe damage to the bearings and possibly to the motor shaft/runner. More importantly, this condition could also result in a fire if the oil, with a flash point of 420 degrees fahrenheit (215.56 degrees centigrade) reaches the pump surfaces which may be as hot as 550 degrees fahrenheit (287.87 degrees centigrade).
Because of these real and serious concerns, each of the two oil pots is equipped with an oil level detector which provides an alarm signal to a control room in the event of an unusual oil level condition. Some concern exists that the detector may generate a high level alarm when, in fact, the system is operating normally, i.e., there is no leakage of water into the oil pump. A major contributor to this potential problem, particularly with respect to the lower oil pot, is the expansion of the oil due to heat entering the oil pot from the reactor cooling pump. A temperature rise of 50 degrees fahrenheit (10 degrees centigrade) in the oil of the oil pot, for example, would result in a volume expansion of approximately 0.6 gallon (2.27 liters) in a 30 gallon (113.56 liters) capacity pot. This is reflected in the rise of the oil level within the pot and the detector of 0.5 inch (1.27 centimeters) or more, and could result in a spurious high level alarm signal.
Recently, a number of nuclear plant operators of pressurized water reactors have experienced a high oil level alarm on the lower bearing oil reservoir shortly after reaching normal operating temperature and pressure. The oil level during the start-up period had gradually increased during heat-up from an initial level of +0.8 inches (+2.0 centimeters) immediately following the across-the-line start. One plant operator noted that upon initial start-up of the pump motor, the oil level abruptly increased from the static zero level to this initial dynamic level. In the attempt to clear the ensuing alarm, two actions were performed. First, the lower oil level alarm switch assembly was adjusted approximately 3/16 of an inch (0.48 centimeters) upward, to the maximum limit of adjustment. This raised the alarm actuation for both the high oil level alarm point as well as the low oil level alarm point for the lower bearing oil reservoir. Prior to this adjustment, the lower oil reservoir alarms were set to occur at approximately +1.25 inches (+3.2 centimeters) above static for the upper alarm and approximately −1.25 inches (−3.2 centimeters) below static for the lower alarm. Second, the vent line between the oil level sight glass and the oil alarm reservoir was disconnected right above the sight glass venting both the alarm reservoir and the sight glass to the containment atmosphere. These directions were taken to clear the oil level alarm encountered during the heat-up of the plant by raising the overall alarm window and provide a more representative oil level based on a known oil inventory in the lower oil pot.
As the plant heat-up continued, the lower oil reservoir again experienced a high level alarm at 1.3 inches (3.3 centimeters) above the static oil level. The plant went on to operate at power and continued to experience a high oil level alarm; with the oil level oscillating between +1.3 and +1.5 inches (+3.3 and +3.8 centimeters) and tracking with changes in ambient and component cooling water temperatures.
The main purpose of the oil level indicating system on the main coolant pumps in pressurized water reactor systems manufactured by Westinghouse Electric Company LLC is for monitoring the oil inventory within the bearing oil reservoir. The original design concept behind the system used on the lower guide bearing oil reservoir on Westinghouse reactor coolant pump motors was a simple transference of indicated oil level from inside the bearing oil reservoir to an alarm switch reservoir and sight glass which is external to the lower bearing oil reservoir (sometimes referred to herein as the oil pot). This approach was taken to eliminate errors due to turbulent flows within the bearing oil reservoir that would be generated if the alarm float switch and sight glass were placed directly into the bearing oil reservoir.
The technical principle for the design of the oil level indicating system of Westinghouse reactor coolant pump motors is a simple force balance accomplished by transference of the pressures from the bearing reservoir to the alarm reservoir generated by the static elevation of a column of fluid. When the fluid is at rest, the height and therefore pressure differences of the oil columns in each reservoir are equal, assuming homogeneous temperatures and fluid properties throughout the system. The alarm reservoir oil level therefore matches the bearing reservoir oil level and oil inventory is easily monitored. Once the fluid is set in motion, however, it's been observed that the pressures in the bearing reservoir can change due to variations in temperature and velocity of the oil. This may unbalance the forces acting on the two oil columns causing unequal heights, or levels, of the columns The oil may also become mixed with air, i.e., aeration; causing changes in oil properties that can tend to contribute to changes in the height of one or both oil columns. Thus, an oil level indicating system must consider these factors in addition to those of the operating environment when used for oil inventory monitoring.